Air con multi-splits: which of 2 approaches is better?

[DragonQ]

I'm planning to get air con in my new house. Use cases are:

Cold snaps: Top-up heating in conservatory during day and kids' bedrooms (2 & 3) at night
Summer: Cooling in conservatory on weekends/holidays, cooling in office (bedroom 4) on weekdays
Heatwaves: Cooling in lounge & conservatory on weekends/holidays, cooling in office (bedroom 4) on weekdays, cooling bedrooms 1-3 at night

The plan of the house is here. We're going to extend the house to add 2 extra bedrooms above the garage in 2-3 years' time. We also may open up the conservatory, dining room, and kitchen into one large area.

I've had 2 quotes from Daikin installers that have different approaches and would like to hear your thoughts about which is better.

Installer 1
- 5kW single split (FVXM50A) for conservatory & dining.
- 4kW outdoor unit (2MXM40A) connected to 2x 2kW indoor units for kids' bedrooms.
- 5.2 kW outdoor unit (3MXM52A) connected to 2x 2kW indoor units for lounge & master bedroom connected to 1x 1.5kW indoor unit for office with pump (pipework through loft).
- An additional 2-way multi-split can be installed in future once extension is done.
- All 3 systems to be run off different ring mains.

Installer 2
- 5.2kW outdoor unit (3MXM52A) connected to 1x 2.5kW indoor unit for conservatory + 2x 1.5kW indoor units for kids' bedrooms.
- 9kW outdoor unit (5MXM90A) connected to 2x 2.5kW indoor units for lounge & master bedroom + 1x 1.5kW indoor unit for office with pump (pipework through loft).
- 2 further indoor units can be connected to the 5MXM90A once extension is done.
- 2 new dedicated circuits for power from garage (via loft).

Installer 2's approach of oversizing the outdoor unit so we can connect more indoor units later is an interesting one but I am not sure how this will affect efficiency, particularly if we only have, say, the office unit on during the day.

Installer 1 thinks 3 outdoor units will work better to reduce pipework and also thinks we need a much bigger unit for the conservatory due to the large number of windows and sky lights, plus it's south facing. If we ever open up downstairs this will help also. But again, for now will the efficiency be poor if the unit is 5kW?

Also, everything I've read suggests single splits are cheaper than multis but that doesn't seem to be true where I live (UK). Installer 3 recommended 6 single splits and wanted to charge 30+% more than the two installers mentioned above.

Based on this, which approach (if any) would work best in your opinion?

 

[DragonQ]

If you have an inverter heat pump it only runs to the capacity of the load, therefore if only one indoor unit is running the efficiency doesn't take a hit.

Not a chance I'd want the outside of my house plastered with loads of heat pumps, it would look like a total mess and is done to save the installer time.

I'd go for the largest capacity multi-split system possible which would work on single phase.

Mitsubishi SCM125ZS-S

This is the Mitsubishi SCM125ZS-S outdoor unit for connecting multi indoor air conditioning units

www.saturnsales.co.uk

Can you explain why outdoor units have minimum power ratings if they only run at the load demanded? E.g. it might say "4.0 to 9.0kW" (can't remember exact numbers). I assumed that means it will always provide 4 kW of cooling even if only 2 kW is demanded (with 2 kW wasted).

The way the house is laid out, none of the outdoor units would be visible from anywhere people would be, with either approach.

 

[DragonQ]

This is very confusing because even recent articles and posts mention how multisplits are much less efficient when not all the indoor units are being used. Are they really all talking about non-inverter units? I thought those weren't even sold any more.

If I understand correctly, longer pipe runs also reduce efficiency, which also makes me weary of over-sizing the outdoor units to later connect 2 more. There'd have to be pipework all the way from one side of the house to the other.

 

[DragonQ]

Your outdoor unit will just do what's needed to exchange the heat with the indoor units demand so will ramp up and down. No idea what you've been reading...

Yes I understand that, but how low can this realistically go without affecting efficiency? Say one indoor unit only requires ~1kW to sustain temperature from a 9kW rated outdoor unit. Would that be much less efficient than an indoor unit requiring ~1kW from a 3kW outdoor unit? Seems to be conflicting opinions on this and I can't find a reliable source, e.g. a datasheet that specifically mentions this scenario.